The other possible values for the
TopologyReliabilityMode
enum are
ATMOST_ONCE
and
EFFECTIVELY_ONCE
.

Instead of “delivery semantics” terminology, the original Topology API for Heron uses “reliability mode” terminology. In spite of the terminological difference, the two sets of terms are synonymous.

Stateful spouts and bolts need to implement the
IStatefulComponent
interface, which requires implementing two methods (both of which are
void
methods):

Remember that stateful components automatically handle all state storage in the background using a State Manager (the currently available State Managers are
ZooKeeper
and the
local filesystem
. You don’t need to, for example, save state to an external database.

Heron topologies with effectively-once semantics need to be stateful topologies (you can also create stateful topologies with at-least-once or at-most-once semantics). All state in stateful topologies is handled through a
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class which has the same semantics as a standard Java , and so it includes methods like
get
,
set
,
put
,
putIfAbsent
,
keySet
,
compute
,
forEach
,
merge
, and so on.

Each stateful spout or bolt must be associated with a single
State
object that handles the state, and that object must also be typed as
State<K, V>
, for example
State<String, Integer>
,
State<long, MyPojo>
, etc. An example usage of the state object can be found in the
example topology
below.

In the sections below, we’ll build a stateful topology with effectively-once semantics from scratch. The topology will work like this:

The plan back then was to make it as easy as possible to build a project that had a number of dependencies for new developers. The main Titan code base built upon , and also made heavy use of ,
Boost
, and a slew of other dependencies that needed to be tested and work on Windows, macOS, and Linux. At the time we decided to make use of the
ExternalProject
feature in
CMake
to orchestrate the build process and this is the core of what many of us refer to as the superbuild. I don’t recall whether we had a dedicated external repository or a mixed code with submodules.

I have been meaning to write some of this up for years, and a colleague encouraged me to do so at a recent workshop, so here you go. Let’s get into some of the detail.

If you learned anything from Ghostbusters (at least in the original) you never cross the streams. In my early days of working with ExternalProject there was a strong temptation to mix the ExternalProject targets building dependencies with normal targets building libraries and/or executables. While some people got it to work some of the time we avoided this practice, and maintained a clear separation of the outer coordinating build, and the inner projects that were built in a sequence as specified by their dependencies.

A strong concept you should bear in mind for any superbuild approach is that you will have an outer build, and this build should only concern itself with building other projects. Building CMake projects is by far the easiest, but it is also possible to drive other build tools, the main challenge is mapping everything from CMake to the external build tool so that you get a consistent result. We took the approach of mirroring the source tree layout in the build tree, so that when you have a VTK directory in the top level of the source tree, there is a VTK build directory in the top level of the build tree.

You may be asking yourself why do we even need to use superbuilds, they just sound like even more complication, and why not just use a package manager. A long time ago I was a
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package maintainer, working on scientific packages and porting to 64 bit processors (which were new back then, I think I am getting old).

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